Debian uses the symlink /usr/bin/sensible-editor to point to the default text editor to use. This is controlled by the Debian Alternatives system, which will work automatically if you want it to. If, however, you want to change the auto-selected "sensible editor", you can do so with this command:

sudo update-alternatives --config editor

This will let you select one of your installed text editors from a list that will look something like this:

In English, this means install into the category x-www-browser, using the symlink named x-www-browser (found in /etc/alternatives), which points to the binary at /usr/local/src/firefox/firefox, and give it a numerical priority of 900. The priority determines which program Debian would select if set to auto. The largest number wins. This is how I added firefox into the list of potential sensible-browser selections.